Given the heterogeneous nature of biological data and their intensive use in many tools, in this paper we propose a semantic data access and integration (DAI) service, based on the grid paradigm, for the bioinformatics domain. This service uses ontologies for correlating different data sets. The DAI proposed in this work is a fundamental component of the ProGenGrid system, a grid-enabled platform, which aims at the design and implementation of a virtual laboratory where e-scientists could simulate complex "in silico" experiments, composing some popular analysis and visualization tools (e.g. Blast and Rasmol) available as Web services, into a workflow. The main goal of the DAI is to provide bioinformatics tools with advanced functionalities and data integration services for heterogeneous biological data banks, such as PDB and Swiss-Prot. A case study of our specialized data access service for locating similar protein sequences is presented.